1. Selective Vaporization Process
The Asphalt Residual Treating (ART) Process is a decarbonizing and demetallization process that has been developed to treat residual stocks and heavy crudes for the removal of contaminants. The process is described in numerous publications, including "The ART Process Offers Increased Refinery Flexibility", R. P. Haseltine et al, presented at the 1983 NPRA Conference in San Francisco. See also U.S. Pat. No. 4,263,128 to Bartholic. The contents of the aforementioned publication and patent are incorporated herein by cross-reference thereto.
The process is a noncatalytic technological innovation in contaminant removal and will typically remove over 95% of the metals, essentially all the asphaltenes and 30% to 50% of the sulfur and nitrogen from residual oil while preserving the hydrogen content of the feedstock. This provides greatly improved cost-effectiveness by producing less unwanted by-products and consuming less energy than competing processes. The ART process also enables the subsequent conversion step in residual oil processing to be accomplished in conventional downstream catalytic processing units.
The ART process utilizes a solid particulate contact material which selectively vaporizes the valuable, lower molecular weight and high hydrogen content components of the feed. The contact material is substantially catalytically inert and little if any catalytic cracking occurs when the process is carried out under selected conditions of temperature, time and partial pressure.
Heavy metals are deposited on the contact material and removed. High molecular weight asphaltenes are also deposited on the contact material, some asphaltenes being converted to lighter products.
The ART process is adapted to be carried out in a continuous heat-balanced manner in a unit consisting primarily of a contactor, a burner and an inventory of recirculating fluidizable contact material. Chargestock is contacted with particles of hot fluidizable contact material for a short residence time in the contactor. In the contactor, the lighter components of the feed are vaporized; asphaltenes and the high molecular weight compounds, which contain metals, sulfur and nitrogen contaminants, are deposited on the particles of the contact material. The metals invariably include vanadium and nickel. Some of the asphaltenes and high molecular weight compounds are thermally cracked to yield lighter compounds and coke. The metals that are present, as well as some of the sulfur and nitrogen bound in the unvaporized compounds, are retained on particles of contact material. At the exit of the contacting zone, the oil vapors are rapidly separated from the contact material and then immediately reduced in temperature to minimize incipient thermal cracking of the products. The particles of contact material, which now contain deposits of metals, sulfur, nitrogen, and carbonaceous material are transferred to the burner where combustible contaminents are oxidized and removed. Regenerated contact material, bearing metals but minimal coke, exits the burner and circulates to the contactor for further removal of contaminants from the charge stock. The selective vaporization process can also be carried out in so-called moving bed mode using pelleted particles of contact material, for example pellets having a diameter of 0.145-0.157 inches and a length of 0.1 to 0.3 inches. See U.S. Pat. No. 4,435,272 to Bartholic et al.
In practice, the metals level of contact material in the system is controlled by the addition of fresh contact material and the removal of spent contact material. A high metals level can normally be maintained without detrimentally affecting performance.
Because the contact material is essentially catalytically inert, very little molecular conversion of the light gas oil and lighter fractions takes place. Therefore the hydrogen content of these streams is preserved. In other words, the lighter compounds are selectively vaporized. The molecular conversion which does take place is due to the disproportionation of the heavier, thermo-unstable compounds present in the residual feedstock.
The hydrogen content of the coke deposited on the contact material is typically less than four percent. Coke production is optimally equivalent to 80% of the feedstock Conradson Carbon Residue content. Heat from the combustion of coke is used internally within the ART system. Surplus heat may be recovered as steam or electric power. No coke product is produced. In contrast, delayed and fluid cokers yield a coke product equivalent to 1.3 to 1.7 times the Conradson Carbon residue.
Generally, metals accumulated on the contact material used in the ART process tend to be less active in forming coke than metals accumulated on cracking catalyst. Thus, the ART process is able to operate effectively when accumulated metals are present on the contact material at levels higher than those which are generally tolerable in the operation of FCC units. For example, the process has operated effectively when combined nickel and vanadium content substantially exceeded 2% based on the weight of the contact material.
Early in the development of the ART process, criteria for suitable contact material were established. See U.S. Pat. No. 4,263,128. These included low activity for catalytic cracking, below 20% conversion by the microactivity(MAT) test, and low surface area, generally below 20 m.sup.2 /g (BET) and preferably in the range of 5 to 15 m.sup.2 /g, and high resistance to attrition. Reference is made to the following U.S. patents of which David B. Bartholic is the inventor: U.S. Pat. Nos. 4,243,514; 4,263,128; 4,309,274; 4,309,274; 4,311,579; 4,311,580; 4,325,809; 4,374,021; and 4,427,538. The aforementioned patents express a preference for microspheres of calcined kaolin clay, in particular microspheres obtained by slurrying naturally-occurring hydrated kaolin clay in water, spray drying to form microspheres and calcining the microspheres at a temperature and for a time sufficient for the clay to undergo the characteristic kaolin exotherm. The resulting microspheres are further characterized in the aforementioned patents as having surface areas in the suitable range, with most of the porosity being contributed by pores having diameters in the range of 150 to 600 Angstrom units. A list of other contact materials appears in U.S. Pat. No. 4,423,514 (supra) at column 5, line 15 to 23. Consistent with the expressed preference for using microspheres composed of kaolin calcined to undergo the exotherm has been the use of such microspheres in commercial practice of the ART process. The ability of such microspheres to remove over 90% of the metal contaminants in heavy feedstocks and provide valuable syncrudes while minimizing catalytic cracking was confirmed. It has been reported, however, that on occasion the microspheres tended to coalesce or agglomerate when the particles of contact material were used over an extended period of time. Agglomeration or coalescence was reported to occur in standpipes, cyclone diplegs and areas of stagnant flow, resulting in loss of fluidization and flowability. See U.S. Pat. No. 4,469,588, Hettinger et al. Two hypotheses, both related to the effect of metals, were advanced. A first was that microspheres of calcined clay were insufficiently porous to soak up the metals. The recommendation that flowed from this hypothesis was to use microspheres that were more porous, in particular microspheres that had a porosity of at least 0.4 cc/g. See U.S. Pat. No. 4,469,588. A related concept underlies the aspect of the invention described in Ser. No. 505,650 now abandoned of Speronello, supra, which proposes the conversion clay into porous mullite by calcining clay past the exotherm to form mullite and free silica, preferably amorphous soluble silica, and then leaching the silica. Leaching of silica results in the creation of pores. The other hypothesis was that vanadium was forming compounds which had relatively low melting points and therefore were in liquid condition during regeneration. The recommendation that flowed from this suggestion was simply to provide means to react vanadium compounds with inorganic compounds such as, for example, those of titanium, calcium, magnesium or rare earth, to form vanadium compounds having higher melting points. See U.S. Pat. No. 4,469,588 (Hettinger et al.)
A desirable feature of an improved selective vaporization process would be the removal of nickel and vanadium from spent contact material. This would allow the reuse of such reactivated contact material in the selective vaporization process. This feature would result in improved process economics. The overall economics of removing metals from spent contact materials for reuse in the process are dependent on the fraction of metals removed and the complexity of the metals removal step. This would also allow the operation of a plant in which any material not generated for sale can be disposed of in land fill.
2. Thermal Conversion of Clay
Kaolin clays are naturally-occurring hydrated aluminum silicates of the approximate formula Al.sub.2 O.sub.3. 2SiO.sub.2.XH.sub.2 O, wherein X is usually 2. Kaolinite, nacrite, dickite and halloysite are species of minerals in the kaolin clay group. It is well known that when kaolin clay is heated in air that a first transition occurs at about 550.degree. C. associated with an endothermic dehydroxylation reaction. The resulting material is generally referred to as metakaolin. Metakaolin persists until the material is heated to about 975.degree. C. and begins to undergo an exothermic reaction. This material is frequently described as kaolin which has undergone the characteristic exothermic reaction. Some authorities refer to this material as a defect aluminum-silicon spinel or as a gamma alumina phase. See Donald W. Breck, ZEOLITE MOLECULAR SIEVES, published by John Wiley & Sons, 1974, pages 314-315. On further heating to about 1050.degree. C., mullite begins to form. The mullitization reaction that takes place when kaolin clay is utilized as the sole source of silica and alumina may be represented by the following equation where the approximate chemical formula for kaolin (without the water of hydration) is given as Al.sub.2 O.sub.3.2SiO.sub.2, and the formula for mullite is 3Al.sub.2 O.sub.3.2SiO.sub.2 : EQU 3(Al.sub.2 O.sub.3.2SiO.sub.2).fwdarw.3Al.sub.2 O.sub.3.2SiO.sub.2 +4SiO.sub.2.
The term represented by 4SiO.sub.2 is the free silica generated as a result of the conversion to mullite. The extent of conversion to mullite is dependent on a time-temperature relationship and the presence of mineralizers, as is well known in the art. The free silica can be amorphous or crystalline and this will also depend on calcination temperature and time and the presence of mineralizers. A high purity kaolin clay can theoretically be converted into about 64% mullite on a weight basis. The free silica formed when kaolin clay is thermally converted into mullite is amorphous when calcination takes place at about 1100.degree. C. Upon heating to temperatures in excess of about 1260.degree. C. silica crystallizes and the amount of silica detectable by X-ray increases with temperature and time. The crystalline silica may be tridymite or cristobalite or both.
Mullite is widely used in ceramic applications such as in the manufacture of refractory grains. For these applications, dense impervious products are needed and porosity is undesirable. See, for example, U.S. Pat. No. 3,462,505.
It is also well known that the reactivity of kaolin clay changes as it undergoes these thermal transitions. See the Breck publication supra at page 315.